Implantable cardiac defibrillators (ICD's) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in a pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. One lead includes at least one defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillation shock pulse from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia. Alternatively, such arrhythmia terminating systems may further include another defibrillation electrode arranged to be positioned in the right atrium and electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered from the parallel connected right ventricular and right atrial electrodes to the conductive housing.
For defibrillation therapy to be effective, the defibrillation pulse provided by the device must be at an output level above the defibrillation threshold. Implantable defibrillators generally provide a biphasic output pulse having a waveform which is shaped based upon a capacitive discharge. Such waveform shapes are generally characterized by a peak starting voltage followed by a capacitive decay. In a biphasic output, the polarity of the pulse is reversed during the duration of the output pulse.
Recently, output pulse waveform shapes, other than those based upon a capacitive discharge have been proposed and possess the potential to lower defibrillation thresholds. Since these devices are fully implantable and rely on battery power, a reduced defibrillation threshold, requiring less defibrillation pulse energy, would lead to extending the useful life of the implanted device. Further, reduced defibrillation energies also have the potential of being less traumatic to the patient.